The present application relates to document boundary determination.
Optical scanners operate by imaging an object, typically in the form of a sheet of paper, document, or other form of medium with a light source. The optical scanner senses a resultant light signal from the medium with an optical sensor array that includes pixel elements generating a data signal representative of the intensity of light impinging thereon for a corresponding portion of the imaged object. The data signals from the array are then processed (typically digitized) and utilized by a utilization apparatus or stored on a suitable medium such as a hard drive of a computer system for subsequent display and/or manipulation.
Various types of photo sensor devices may be used in optical scanners. For example, a commonly used photo sensor device is the charge coupled device (CCD), which builds up an electrical charge in response to exposure to light. The size of the electrical charge built up is dependent on the intensity and duration of the light exposure. In optical scanners, CCD cells are typically arranged in linear arrays. Each cell or “pixel” has a portion of a scan line image impinged thereon as the scan line sweeps across the scanned object. The charge build up in each of the pixels is measured and discharged at regular “sampling” intervals.
The image of a scan line portion of a document is projected onto the scanner's linear sensor array by scanner optics. In CCD scanners, the scanner optics typically comprise an imaging lens which typically reduces the size of the projected image from the original size of the document. Pixels in a scanner linear photo sensor array are aligned in a direction perpendicular to the “scan” direction, i.e. the paper or scanner movement direction for scanning of the image.
At any instant when an object is being scanned, each pixel in the sensor array has a corresponding area on the object which is being imaged thereon. This corresponding area on the scanned object is referred to as an “object pixel.” An area on a scanned object corresponding in area to the entire area of the linear sensor array is referred to as an “object scan line” or “scan line.” For descriptive purposes, a scanned object is considered to have a series of fixed adjacently positioned scan lines. Scanners are typically operated at a scan line sweep rate such that one scan line width is traversed during each sampling interval.
Some optical scanning machines include an automatic document feeder for feeding a document past the optical array. Other optical scanners machines are known as “flat-bed” scanners, wherein a document is placed on a fixed platen for scanning, which occurs by moving the sensor array relative to the fixed document.
It is advantageous in various applications to sense the location of a document edge. In a printer, for example, the print area differs depending on whether the printing is on envelopes, name card paper, letter sized paper, and so on. The prediction of the print area assists in driving the print head. The print area can be identified by sensing the media edges. By identifying the document area, proper clipping can be made on both sides when printing. In a scanner, detection of the document edges can assist by placing the image area properly on the page, and by reducing the scan memory size by clipping the empty regions. Also, by detecting the edge position in the direction of document movement, the document skew can be estimated and used to redirect the scanned image in print. This will produce a more pleasing output from the scanning process. In a copier, sensing the size of a document permits scaling of the input document to the maximum size that will fit on an output page. In addition, multi-function machines combine in a single machine the functions of printing and optical scanning with automatic document/sheet feeders.
If a document is misaligned with respect to the optical sensor, the resultant image is similarly skewed. Because the contents of a document page are usually aligned with the page itself, a skewed page usually results in a misalignment with the optical sensor.
Pasco et al., U.S. Pat. No. 5,818,976, disclose a system for skew and size/shape detection of a document. The system performs the following basic steps, (1) detects points near the edge of the page image, (2) fits lines to establish a closed contour, and (3) defines a polygon with sides coincident with the lines of the closed contour. The polygon defines the size and shape of the page image. With respect to detecting the edges of the page the system uses a background (platen backing cover) that contrasts well with the page, e.g., a black (or gray or patterned) background and white documents. Then the system analyzes the image to determine the edges of the image. Unfortunately, this requires specialized hardware to determine the edge of the image and thus is unsuitable for general purpose scanning devices. If a contrasting background is not used, Pasco et al. suggest the use of electromechanical switches or optical switches arranged to sense the location of edges of each page in conjunction with scanning. Likewise, this requires specialized hardware to determine the edge of the image and thus is unsuitable for general purpose scanning devices.
What is desired, therefore, is a system that can determine the general bounding region of a document without additional specialized hardware.